1. Field of the Invention:
The present invention relates to a sliding member and a fluid dynamic pressure bearing apparatus using the sliding member.
2. Description of the Related Art:
In the recent years, a fluid dynamic bearing (FDB) using an oil as fluid is used as the bearing for a motor. The fluid dynamic bearing has a long service life and causes less noise as compared with a ball bearing, and thus the fluid dynamic bearing is adopted in a hard disk drive, a heat exhaust fan, etc. In some cases, a high-lubricity coating is applied on a sliding surface of the fluid dynamic bearing, in addition to a dynamic-pressure generating groove formed on the sliding surface. As such a high-lubricity coating, a diamond-like carbon layer (DLC layer) is used in many cases. The diamond-like carbon layer generally has excellent frictional wear resistance property and high hardness.
On the other hand, a diamond-like carbon layer which has a high hardness (high-hardness diamond-like carbon layer) has high density and thus has high film stress, and has low adhesion property with respect to a base member or substrate. This consequently causes such a problem that the high-hardness diamond-like carbon separates from the substrate. Conventionally, a base layer formed of a metal such as chromium is provided for the purpose of improving the adhesiveness between the diamond-like carbon layer and the substrate. However, although the base layer made of metal is capable of preventing a low-hardness diamond-like carbon layer having a hardness of about 10 GPa from separation, the base layer made of metal is not capable of sufficiently preventing a high-hardness diamond-like carbon layer (hardness of about 30 GPa, for example) having excellent frictional wear resistance property from separation. For this reason, Japanese Patent Application Laid-open No. 2004-10923 discloses providing a hardness-gradient diamond-like carbon layer with varying hardness in the thickness direction thereof, in addition to a metal base layer. Further, Japanese Patent Application Laid-open No. 2008-81630 discloses a sliding member having a multi-layered film in which a low-hardness diamond-like carbon layer and a high-hardness diamond-like carbon layer are alternately stacked, in addition to a metal base layer and a diamond-like carbon layer with increasing hardness in a continuous or stepped manner.